1. Field of the Invention
The present invention relates to an exhaust gas cleaning system of an internal combustion engine, in particular, to a method of regenerating a particulate filter.
2. Description of Related Art
Lately, improvement in exhaust emission of an internal combustion engine mounted on a vehicle and the like is required. Specifically, exhaust particulate matters such as soot or a soluble organic fraction included in the exhaust gas discharged from a compression-ignition type diesel engine running on diesel oil should be removed, as well as carbon oxide, hydrocarbon and nitrogen oxide. Therefore, a particulate filter made of a porous material is disposed in an exhaust passage of the diesel engine for collecting the exhaust particulate matters included in the exhaust gas.
When the exhaust gas passes through porous filter walls of the particulate filter, the particulate matters included in the exhaust gas are filtered at surfaces or pores of the filter walls. If an excessive quantity of the particulate matters is collected, flow resistance at the particulate filter may increase. As a result, back pressure of the engine may be increased, and the engine output may be decreased. Therefore, ability of the particulate filter to collect the exhaust particulate matters should be recovered by regenerating the particulate filter. The particulate filter is regenerated by eliminating the collected exhaust particulate matters.
A particulate filter having an oxidation catalyst such as platinum thereon can be regenerated during the operation of the engine with the use of an oxidization effect of the oxidization catalyst. For instance, a post-injection for injecting fuel in an expansion stroke of the engine is performed at a predetermined timing in order to provide the fuel to the particulate filter. The temperature of the oxidization catalyst is increased with the use of heat, which is generated, by combusting the fuel. Thus, the collected particulate matters are eliminated. Otherwise, the timing of the normal fuel injection is retarded to decrease an efficiency of the engine. Thus, waste heat, which is not converted into motive energy, is increased, and the temperature of the oxidization catalyst is increased with the use of the waste heat. Thus, the particulate matters depositing in the particulate filter are combusted and eliminated.
In a method disclosed in Japanese Patent Unexamined Publication No. H07-332065, timing for regenerating the particulate filter is determined based on a pressure difference between an inlet and an outlet of the particulate filter. More specifically, it is determined that the particulate filter should be regenerated, if the pressure difference exceeds a predetermined value. The above method is based on the fact that the pressure difference between the inlet and the outlet of the particulate filter increases as the flow resistance increases. If the regeneration of the particulate filter is performed frequently, fuel consumption may be increased. Therefore, preferably, the predetermined value should be set as high as possible in a permissible range in order to reduce the frequency of the regeneration.
However, in the above method, the pressure difference decreases if the flow rate of the exhaust gas passing through the particulate filter decreases. In such a case, there is a possibility that the quantity of the collected particulate matters cannot be measured with adequate accuracy.
Therefore, there is a possibility that the detected pressure difference does not exceed the predetermined value even if the quantity of the collected particulate matters is above an upper limit, above which the regeneration of the particulate filter is required. In such a case, when it is determined that the regeneration should be performed afterward, the quantity of the collected particulate matters may be excessive. As a result, the particulate matters may burn rapidly in the regeneration, and the particulate filter may be heated up to an abnormally high temperature, damaging itself.